Secretary of State Hillary Rodham Clinton says she is "very concerned" about violence between Sudan and South Sudan.
She is urging both countries to halt fighting, withdraw their troops from border areas and resume peace talks.
South Sudan's President Salva Kiir said Tuesday that Sudan had "declared war" on his country after it dropped eight bombs into the south. The south gained independence from the north last year.
Clinton acknowledged the withdrawal by the world's newest nation from the oil-rich border town of Heglig. She said that presented an opportunity for negotiations with the north, which the U.S. considers a state sponsor of terrorism.
The neighbors dispute borders and how to share oil revenue.
Clinton said the issues were tough but that talks were preferable to war.